Drug delivery devices, which allow multiple dosing of the required dosage of liquid drug and administration of the liquid to a patient, are well known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe.
Injectors of this kind must meet a number of requirements to meet user needs. These devices need to be robust in construction, yet easy to use both in terms of the manipulation of the parts and understanding by a user of its operation. In the case of those with diabetes, many users will be physically infirm and may also have impaired vision. Where the injector is to be disposable rather than reusable, the injector should be cheap to manufacture and easy to dispose of (preferably being suitable for recycling).
WO 9114467 A1 discloses a dispensing device, comprising a drive mechanism having a telescopic piston rod consisting of a first and a second threaded member having equi-spaced threaded segments with non-threaded segments therebetween. The device further comprises a dose setting sleeve, which surrounds the second threaded member being coupled for rotation and is threadedly engaged with the device shell. The threads of the three said components are of the same lead. The design of this device requires a body length to plunger length ratio of about 1:1 in order to allow dispensing of relatively large doses. However, simple and safe correction of a set overdose by the patient remains unresolved without either dispensing the set amount of fluid or dismantling the cartridge.
WO 9938554 A2 teaches an injection syringe for apportioning set doses of a medicine from a cartridge wherein a drive mechanism comprising a unidirectional coupling (ratchet) is disclosed which allows correction of a set overdose without dispensing a set amount of fluid or requiring the dismantling of the cartridge. The device is further designed so that an initial reluctance has to be overcome before the unidirectional coupling can be rotated during dose dispensing.
WO 0195959 A1 discloses an injection device for injection of set doses of medicine from a cartridge, in which syringe a dose is set by screwing a nut up along a threaded piston rod, whereby a dose setting drum, which carries on its cylindrical surface along a helix a scale of which a number corresponding to the set dose is shown in a window in the housing of the syringe, and an injection button, which is elevated over the end of the syringe, are moved axially a distance which is larger than the axial movement of the nut. A gear wheel transmission is established between the nut and the injection button such that the gearing makes the movement of the button larger and the force, which has to be exerted on the injection button, is correspondingly reduced.
Accordingly, the problem to be solved by the instant invention is to reduce the actuation (injection) force required during drug dispensing especially when dispensing large doses whilst maintaining a compact design and thereby improving user handling, and providing the user with a means to intuitively, safely and easily correct a set dose.